1. Field of the Invention:
The present invention relates to motor protection circuits for direct current motors, and more particularly, to such circuits in which dynamic braking voltages and dynamic braking currents which exceed predetermined safe levels are precluded while at the same time the application of dynamic and mechanical braking simultaneously for more than a preset time interval is avoided.
2. Description of the Prior Art:
Direct current motors, and in particular those which are used to power vehicles of various types, such as trains, cars and trucks, and in operating elevators, winches and other machinery, are subject to severe mechanical damage if improperly operated during those periods of time when dynamic braking is applied. During such dynamic braking, when the motor is acting as a generator and is absorbing energy from the kinetic energy of the moving load and dissipating it in resistors, excessive voltages and/or currents may be generated. Through improper use of the motor control mechanism, these excessive voltages and/or currents may reach and exceed levels at which electric arcs are drawn between various parts of the motor and/or the frame, melting metal and destroying insulation. These electric arcs, commonly known as flashover, can produce serious damage requiring extensive and costly repairs and loss of effective operating time.
The prior art, as exemplified by U.S. Pat. Nos. 2,656,501, 2,926,759, and 2,933,350, is generally cognizant of systems which automatically regulate the application of dynamic and mechanical or electro-pneumatic braking systems so as to achieve a smooth and efficient overall braking characteristic while at the same time preventing damage to system components. While these systems are generally effective in achieving desired braking characteristics, the flashover problem continues to exist and in many situations excessive voltages and/or currents are permitted to develop beyond the limits which can be tolerated.
A related problem has also been recognized in the prior art with respect to the common or simultaneous actuation of dynamic and mechanical braking systems for excessively long periods of time. In connection with rapid transit systems, for instance, under normal braking operation the mechanical brakes should be applied only when the train speed has slowed to the point that dynamic braking will soon become ineffective. When both mechanical and dynamic braking systems are simultaneously applied while the train is travelling at relative high speed, the wheels can lock and slide resulting in the production of flat spots.
The tendency to exhibit wheel locking and the damaging flashover which can occur by improper application of dynamic braking are substantial disadvantages which have long plagued systems conventionally used in the past.